The ideal surgical tool would be as non-invasive as possible, by making the smallest cuts and removing the least amount of material possible. Since the invention of the laser, people have envisioned the use of laser based scalpels that are capable of removing or cutting away tissue with sub-micron accuracy. However, the problem to date with methods of laser surgery has been the deleterious collateral damage to surrounding tissue, which has limited the widespread use of lasers as a replacement for mechanical surgical tools.
Material ablation requires that the energy delivered by a laser to a material results in the irradiated volume of the material gaining sufficient translational energy to cause it to be removed. A number of mechanisms have been utilized, including “phase explosion” [1,2] and photomechanical spallation [1,3,4]. In the case of photomechanical spallation, the energy deposited must not exceed the vaporization threshold, and must be deposited quickly enough that shockwaves due to sudden thermal expansion lead to mechanical breakdown and ejection of the material. However, photomechanical spallation is notoriously inefficient, and it has been found that “ . . . for most tissues, material removal using IR laser pulses cannot be achieved via a laser-induced spallation mechanism.” [1]
In the case of “phase explosion”, enough energy must be deposited to superheat the irradiated volume above the vaporization temperature of the material, leading to homogeneous nucleation and ejection of the material. The faster the heat is delivered to the material the faster the ablation process and the less amount of energy is lost by heat transfer to the surrounding material. Thus the degree of collateral damage depends strongly on the laser pulse width [1,5]. For long laser pulses (defined here as greater than 10 ns), there is significant heat transfer to the surrounding material. In the case of biological materials, the temperature gets so hot that the material actually burns. This latter process not only damages the surrounding tissue but actually blocks healing.
This dramatic effect is shown in FIG. 1, in which long pulses from conventional lasers are compared to short pulse ablation mechanisms in cutting bone as will be discussed below. Water cooling in combination with long laser pulses can help prevent the charring effect by excluding oxygen that is required for combustion. However, the material must still get hot enough to ablate in the irradiate volume and the excess heat which accumulates in the surrounding material leads to the tissue damage is the same. Excess heating of this magnitude still severely harms the adjacent tissue and thus has prevented the general use of conventional long pulse lasers in medical surgery.
It should be further noted that the time scale for heat deposition depends on both the time it takes for the absorbed laser energy (photons) to be converted to heat and the pulse duration of the incident light. The rate of lattice heating is a convolution of these two effects. The longer it takes the incident laser energy to be converted to heat the more time thermal diffusion and thermal expansion have to transfer energy to surrounding material and increase collateral damage. In the case of short laser pulses, it is possible to deposit the laser energy much faster than the ablation process, but realization of this is complicated by other deleterious effects as discussed below.
The threshold for ablation depends strongly on the energy deposited per unit volume. For a given amount of laser energy, the smaller the amount of material absorbing the light, the higher the temperature of the lattice rises in the irradiated zone, and the less total heat is available for transport to adjacent material to cause heat induced damage. In other words, the smaller the optical penetration depth, the less energy is required to achieve a particular temperature for ablation and the less total heat transferred to the material. The objective in minimizing collateral damage in laser cutting is to maximize the energy absorbed per unit volume or mass to increase the ablation efficiency.
To ensure a short absorption depth, excimer (UV) lasers can be used because all materials strongly absorb light in the UV to VUV wavelength range (here defined as micron to sub-micron absorption depths). However, this wavelength range leads to radiation damage by UV photochemistry and ionization of the material. This photochemistry is undesired in many applications and is a particularly serious problem in most medical and dental applications where ionizing effects and collateral photochemistry must be avoided in order to promote healing.
The alternative to the use of UV lasers is to use femtosecond (fs) laser pulses. With short enough pulses, it is possible to achieve high enough peak powers to drive multiphoton absorption processes that lead to strong absorption within the laser focal volume, even at wavelengths where the material is nominally transparent. However, the peak power must be raised to a level that leads to the ionization of the material's constituent atoms or molecules. The liberated electrons absorb light at all wavelengths and rapidly convert this energy to heat through a process referred to as avalanche ionization [6,7]. At high enough densities, the electrons and parent ions form a plasma that can further increase the absorptivity and further localize the heat deposition. In this short pulse limit, there is also rapid heat deposition. However, this process creates highly reactive intermediates. For solid state materials such as metals, this ionization process has no consequence as the ions are quickly discharged. In the case of biological materials, the presence of ions creates the same effect as highly ionizing x-ray radiation. Even in the absence of excessive heating in this short pulse limit of femtosecond durations, the effect of ion formation leads to damage that blocks healing and is unacceptable for most medical applications.
This problem has been identified by control studies of live animals in which it was found that even though femtosecond laser pulses cut without excess heating, the healing process was not invoked. There was sufficient damage to adjacent cells to kill them but not enough to trigger normal healing mechanisms. A signalling protein had to be used to induce healing as disclosed in co-pending U.S. patent application Ser. No. 60/704,905 filed Aug. 3, 2005 entitled “Hybrid ultrafast laser surgery-growth factor stimulation for ultraprecision surgery with healing”. Note the effect of ionizing radiation from multiphoton absorption may also be significant in other non-biological materials. In the case of sensitive materials such as semiconductors, insulators, catalysts, etc., the ion induced chemistry will also lead to highly undesirable changes in material properties, such as the creation of surface states/defects. This effect again is most undesirable in any kind of medical or dental application. In the case of dental applications, the formation of highly reactive ions will have the potential for forming organic mercuric compounds from ablation of amalgam fillings that are incredibly toxic.
Another alternative is to tune the laser wavelength to maximize the energy localization into short lived excited states, such as vibrational transitions in the mid-infrared (mid-IR) wavelength range. Strongly absorbing vibrational modes, with micron to sub-micron absorption depths, can be found in biological and most other materials. By targeting these short-lived excited states thermal energy can be quickly transferred to a small irradiated volume, without the deleterious effects of ion generation and photochemistry. However care must be taken in the choice of both pulse duration and energy in order to achieve efficient, collateral damage free ablation.